Z. Yang, D. Tang, C. Goode
Cirrus Materials Science,
New Zealand
Keywords: anodising, white surfaces, durable coatings, aluminium alloy
Summary:
The authors present the novel Cirrus Lustre™ white anodising process, which creates a durable, natively white surface on Aluminium alloys. Cirrus Lustre™ comprises a single step anodising process that produces a porous alumina structure with light scattering intersections, from which, most incident light scatters and reflects presenting a white appearance. The novel anodising process develops a graded alumina porosity throughout the coating thickness to form light scattering points, which may present differently depending on the Aluminium alloys. A unique seal process completes the coating to produce a robust white surface. Conventional approaches to create white surfaces on Aluminium alloys include paints and powder coats, however both processes require volatile organic compounds, and produces white surfaces that are not durable. Electrolytic colouring by incorporating Titania nanoparticles, as a dye into conventional anodic structures is a current research direction. Unfortunately, electrolytic colouring requires an anodising step, a colouring step and a seal step, while the translucent alumina structures allow the substrate to remain visible limiting the degree of lightness available. As the colour is induced by the anodic structure, Cirrus Lustre™ offers an eco-friendly, VOC, and nano particle free one step process to create uniform white surfaces. Cirrus Lustre™ white presents an L* lightness >92, a* and b* close to zero with ΔE less than 0.5. The as anodised white surface is naturally durable and corrosion resistant having a Vickers hardness which increases from about 200 near the coating surface to over 800 near the substrate and a corrosion current density below 0.06μA/cm2 from Tafel analysis. A novel electrophoretic silica alumina seal process completes the Lustre process. Cirrus has specifically designed the seal to integrate with the surface alumina during low temperature curing to develop boehmite structures which increases the surface mechanical strength. The sealed white surface after curing becomes super hydrophobic, with contact angle with DI water above 150°, creating a surface which is chemically resistant, corrosion resistant, and scuff resistant for applications such as consumer electronics, aviation, automotive and architectural industries.